The basal ganglia are a group of nerves cell clusters, called nuclei, located deep in the lobes of the brain collectively referred to as subcortical nuclei. The different ganglion (nuclei) are labeled in the picture below and include the: caudate nucleus, putamen, globus pallidus, subthalamic nuclei, substantia nigra and nucleus accumbens, which is not included in this picture. The general outline of these subcortical nuclei looks similar to the outline of the limbic system because both systems surround the lateral and third ventricles found in the core of the brain.
The basal ganglia are important to the execution of smooth movements from start to finish. They also play a role in learning routine movements, including bad habits, such as obsessive compulsive disorders, as well as cognition and emotion. As far as movement is concerned, one of their functions is to inhibit excitation of muscles to prevent overreactions. The ganglia get their signals from many different areas in the lobes of the brain, as well as from the midbrain which is the top of the brainstem.
Glutamate, is an excitatory neurotransmitter and the most abundant used by the basal ganglia. Acetylcholine is also used and is, likewise, excitatory. In addition to the excitatory glutamate and acetylcholine neurotransmitters, the nuclei of the system also use GABA and dopamine which are inhibitory. Although they are less abundant compared to the excitatory transmitters, the input from the inhibitory neurotransmitters plays a key role in smooth execution of movement.
The basal ganglia are most noted for their involvement in Parkinson’s and Huntington’s disease. While the two conditions share certain signs and symptoms, they are notably different. For example, Parkinson’s disease primarily affects the substantia nigra resulting in decreased production of the inhibitory neurotransmitter dopamine. One of the characteristic signs of Parkinson’s disease is that patients have a hard time initiating movement to get going.
Huntington’s disease primarily affects the caudate nucleus and putamen. In contrast to decreased dopamine seen in Parkinson’s, Huntington’s disease is associated with excess glutamate and subsequent excitotoxicity. I will cover Huntington’s disease more as this site develops. As for movement, patients with Huntington’s disease have a hard time preventing unwanted movements. What these conditions share in common, is that in advanced stages they can both significantly impair cognition.
More recently, the basal ganglia have been implicated in multiple sclerosis (MS). Instead of the characteristic supratentorial periventricular and perivenular hyperintensity signals associated with the white matter of myelinated nerves seen in MS, they show up as hypointensity signals found in the grey matter nuclei, such as the basal ganglia, that are located deep within the lobes of the brain along the lateral ventricles.
The difference between hyper and hypointensity signals seen on MRI in MS has to do with fluid content and the ability of fluids to diffuse through different densities and types of tissues that surround them. Thicker fluids don’t diffuse as well as thinner fluids. The differences in thickness and diffusibility affect appearances on MRI scans. Hyper, hypo and isointensity signals will be covered further in future pages. They are the focus of current research and rapidly adding to our understanding of neurodegenerative processes because radiologists have the technology now to see things they couldn’t before.
The signs and symptoms of basal ganglia involvement include tremors, involuntary movements, weakness and spasticity. These signs and symptoms are common in Parkinson’s disease and multiple sclerosis.